HIV/AIDS is a global pandemic with 35 million individuals living with HIV infection and approximately 39 million have died from AIDS worldwide. The objectives of this project are to define the unique epidemiological, clinical, virologic, and immunologic features of HIV infection in developing countries, to determine the viral kinetics associated with sexual transmission, and to characterize the molecular strains of HIV internationally for infectiousness and progression of disease. Accurate methods of estimating HIV incidence from cross-sectional surveys are critical to identifying groups at high risk of infection, monitoring the epidemic, and determining the population level impact of prevention efforts. We determined that we could use a combination of antibody titer and antibody avidity assays with CD4 and viral load testing where the mean duration of individuals that appear recently infected is 159 days. In comparative incidence studies, incidence estimates based on cross-sectional testing were nearly identical to the observed incidence in three longitudinal cohorts. We further noted that infection with subtype D and antiretroviral treatment (ART) or ART use during PrEP diminish antibody responses seen in infection. In analyzing samples from the CAPRISA 004 trial, we observed that women in the treatment arm had a marked delay in antibody avidity maturation compared to the women in the placebo arm. These factors have implications to the use of antibody assays for identification of recent infection and vaccine development. The Johns Hopkins Emergency Department has served as an observational window on the HIV-epidemic in inner city Baltimore for over 25 years. We recently analyzed 7 discrete identity-unlinked serosurveys conducted on 18,240 untargeted adult JHH-ED patients between 1987-2013 for demographic trends in HIV prevalence, cross-sectional incidence estimates, viral load and HCV prevalence. HIV prevalence in 1987 was 5.2%, peaked at 11% from 1992-2003, and then declined to 5.6% in 2013. Proportion of undiagnosed HIV declined over time from 77% in 1987 to 12% by 2013. HIV incidence estimates in 2001 were 2.1% and declined steadily to 0.16% by 2013. Proportion of HIV+ individuals with viral suppression increased steadily from 23% in 2001 to 59% by 2013. Consistent with increasing viral suppression, 80% of 214 HIV+ individuals surveyed in 2013 had antiretroviral drugs detected in their sera, a marked increase from 2007 (27%). However, HCV in this population remained at 18-19% from 1988 until 2007 and declined only slightly to 14% by 2013. To investigate HIV transmission dynamics at the community level, we applied three analytical methods to data collected from 14,594 individuals living in 46 communities in Rakai, Uganda. Spatial clustering analysis indicated that individuals who lived in households with individuals with incident HIV or prevalent HIV were 3.2 times more likely than the general population to be HIV-positive. Spatial clustering outside households was relatively weak, however, and was confined to distances of less than half a kilometer. Phylogenetic analyses of gag and env genes suggest that chains of transmission frequently cross community boundaries. Using the locations of self-reported sexual partners, we estimate that 39% of new viral transmissions occur within stable household partnerships, and that among those infected by extra-household sexual partners, 62% are infected by sexual partners from outside their community. The results suggest that HIV introductions into communities are frequent and are likely to play an important role in sustaining HIV transmission in the Rakai District. Consequently, to halt the spread of HIV, prevention efforts will need to be implemented at spatial scales broader than individual communities, and key populations that are likely to introduce HIV into communities will need to be targeted. We have expanded our use of ultra-deep pyrosequencing to identify HIV superinfection and describe its effects on the pandemic. In a study of post-partum women in Malawi we found eight confirmed SIs in women who infected their infants via breast feeding with a rate of SI of 7.5/100pys, and five confirmed SIs in the non-transmitters for a rate of SI of 4.4/100pys (p=0.78), suggesting that SI was common in this group but did not increase transmission via breast feeding. The occurrence of HIV-SI was also shown to increase broadly reactive neutralizing antibody and further studies are warranted to examine the implications for vaccine research. We undertook a study to characterize prevalence of HCV, HIV/HCV co-infection and the HCV care continuum among IDUs in 15 cities throughout India. 14,450 IDUs who were &#8805;18 years of age and reported drug injection in the prior 2 years were surveyed. The median weighted HCV prevalence across cities was 41.1%; HIV/HCV co-infection prevalence ranged from 2.0 to 28.5%. Of the 5,777 IDUs with HCV infection, only 5.6% had been previously diagnosed, 2.7% were linked to care, 1.4% had ever been treated and 0.38% exhibited sustained virologic response. Engagement in HCV care was significantly better among participants living in states with established injection drug use epidemics. The high burden of HCV and HIV/HCV co-infection coupled with low-access to HCV testing and treatment highlights the urgency to include India in the global HCV agenda especially given recent advances in HCV treatment.